1. Field of the Invention
The present invention relates to an optical system for scanning light beams, and an image recording exposure device for recording an image on an exposure surface by performing simultaneous multiple main-scans on the exposure surface.
2. Description of the Related Art
In a conventional image recording device, an image is recorded on a photosensitive material by disposing at a predetermined distance scan-units facing a portion of the circumferential surface of a cylindrical drum (outer drum), carrying out main-scanning on a surface of the photosensitive material by high speed rotation in a state in which the photosensitive material is wrapped around the circumferential surface of the cylindrical drum and, during this high speed rotation of the drum, carrying out sub-scanning on the surface of the photosensitive material by moving the scan-units in an axis direction of the drum while maintaining the distance of the scan-units. In this image recording device, it has been considered that a plurality of light sources (a light source having a broad light emission area structured by point light sources arranged in at least one direction) is arranged at the scan-units, and light beams transmitted from the light sources are arranged in a sub-scanning direction to thereby record an image on the photosensitive material while forming main-scan lines simultaneously. As a result, high speed processing is made possible.
Further, the aforementioned light sources are useful when a light source which emits light with high energy is needed, such as with a thermal photosensitive material.
In such an image recording device, when a distance between the spots of respective light beams is changed (switched) in accordance with a desired resolution, it becomes necessary to change the magnification of an exposure lens which is arranged at each scan-unit.
During this process, when the magnification of the exposure lens is made higher, a converging angle that is formed by light beams which are condensed (focused) on an image recording surface (scanning surface) becomes larger, and a depth of focus which is acceptable for the exposure decreases.
FIGS. 9A to 9D show an example of an optical system for explaining the change of the depth of focus.
As shown in FIG. 9A, a divergent light which is emitted from a light source 100 is changed to a parallel light at a first lens 102, and is focused through a second lens 104 onto a scan surface 106. At this point, the light beams form a converging angle xcex81.
FIG. 9B shows an enlarged view of a focus point at this time. An allowable spot diameter (beam diameter) d1 is defined, and thus a width (focal depth L1) for obtaining this allowable spot diameter d1 is determined.
FIG. 9C shows a state in which the magnification is made higher by moving the second lens 104 in the direction of an optical axis (the imaginary line position in FIG. 9C shows the state of FIG. 9A).
At this time, it is noted that a converging angle xcex82 formed by the light beams which are condensed by the second lens 104 is greater than the converging angle xcex81.
FIG. 9D shows an enlarged view of the focus point at this time. A width (depth of focus L2) for obtaining a defined allowable spot diameter d2 (which equals d1) is smaller than L1.
In order to solve this problem, removing any other expected factors that may cause errors (even when the depth of focus is low) can be considered. The any other factors that may cause errors include eccentricity of the outer drum, an amount by which image surfaces of the scan-units are curved, and the like.
Mitigation of such errors depends on accuracy with which mechanical parts are manufactured, accuracy with which the same are assembled, and complicated control of the device, so that the manufacturing cost of the device becomes high. This is particularly unsuitable for image recording devices which are desired to be manufactured inexpensively.
Due to the introduction of AF (auto-focus) mechanisms, the aforementioned accuracy requirement can be eased. However, the AF mechanism is deficient in reliability because of a problem with responsiveness. Moreover, because control of the AF mechanism itself is complicated, it is difficult to solve the problem of high manufacturing costs.
In view of the aforementioned facts, an object of the present invention is to obtain a scanning optical system having a simple structure, capable of preventing depth of focus from decreasing when magnification of the optical system is changed, especially when the magnification is increased, and capable of scanning while maintaining a range of acceptable spot diameter for all magnifications.
Another object of the present invention is to obtain an image recording exposure device which has an optical system for simultaneously scanning multiple light beams and in which, even with a change of resolution, the depth of focus can be maintained without causing a reduction of light amount, such that a spot diameter being scanned on a recording surface is kept within an acceptable range.
A first aspect of the present invention is an optical system for use in scanning a surface, the optical system comprising: a light source which emits a light beam; a group of lenses which condenses the light beam emitted from the light source to a surface to be scanned; a magnification changer for changing magnification of the group of lenses; and an aperture provided on an optical path of the light beam and having an opening in which only a portion of the light beam is transmitted therethrough.
A second aspect of the present invention is an optical system for use in scanning a surface, the optical system comprising: a light source including a broad light emission area comprising point-like light sources arranged in at least one direction; a lens group which condenses a light beam emitted from the light source to a surface to be scanned; a magnification changer which moves at least one lens of the lens group along an optical axis direction of the light beam for changing magnification of the lens group; and an aperture provided on an optical path of the light beam and including an opening which transmits only a portion of the light beam therethrough.
In accordance with the first and second aspects of the present invention, an amount of the luminous flux corresponding to an amount of widening of a converging angle at a converging point, due to an increase of magnification, is shielded by the aperture. Thus, the luminous flux that reaches the scanning surface forms almost the same converging angle as before the increase of magnification. Consequently, depth of focus can be prevented from decreasing.
For this case, intensity distribution of a laser beam L at a position at which an aperture is arranged is shown in FIG. 1. The intensity distribution has a mountain-like characteristic whose central portion is the highest intensity and which deteriorates toward a periphery (foot of the mountain). Thus, if a peripheral edge portion of this intensity distribution is shielded by an aperture, the total light amount does not decrease in direct proportion to the area shielded by the aperture.
As shown in FIG. 2, assuming a Gaussian distribution, a radius of an opening portion 28 of an aperture arranged on an optical path of r, a light amount of 1/e2, and the radius r and a light transmissivity T each normalized as 1, when the aperture opens at a radius r other than 1, a light amount ratio of the radius r to the light transmissivity T has a relationship close to a direct proportion. When the radius r is xc2xd (i.e., the area is xc2xc), the light amount is also xc2xd.
If the magnification is increased, regardless of the shape of the opening portion, a total light amount in a broad area direction is subjected to basic geometry, a converging spot diameter becomes smaller, and the light amount in unit area increases. If the broad area direction corresponds to the sub-scanning direction, the converging spot diameter in the sub-scanning direction becomes smaller, and thus the peak value of the light amount increases substantially linearly (when the converging spot diameter is xc2xd, the peak value is almost double). Thus, the decrease of the light amount is offset, so the light amount is hardly affected by the aperture.
In order to record an image at higher resolution, the line width must be narrower by an amount in accordance with a desired resolution. When a broad area light source is used, the converging spot diameter becomes smaller. By contrast, in the case of a coherent light source (point light source), when the aperture is made larger, the spot diameter only changes a little, and it is difficult to obtain the effect described above. Further, the peak value of the light amount decreases in correspondence with an amount by which the aperture is closed.
In accordance with the first and second aspects of the present invention, multiple light beams are irradiated from the light source at one time. The aperture can be arranged on a far-field pattern of the light beams or in the vicinity thereof.
By arranging the aperture on the far-field pattern of the light beams or in the vicinity thereof, each of the light beams is uniformly shielded by the aperture at the peripheral edge portion of the luminous flux. Accordingly, differences between light amounts of the respective light beams are not caused.
In accordance with the first and second aspects of the present invention, the aperture may be arranged at a downstream side of any lenses which move along the optical axis direction during a change of the magnification, and so the opening area of the aperture can be made constant.
The opening area of the aperture can be made constant because the region of the luminous flux is not changed.
Further, in accordance with the first and second aspects of the present invention, the aperture may be arranged at an upstream side of the lenses which move along the optical axis direction during a change of the magnification, and the opening area of the aperture can be changed in accordance with the changed magnification.
Because the aperture is arranged at the upstream side of the lenses which move along the optical axis direction, the region of the luminous flux is changed. Therefore, in order to fix the proportion of the region of the luminous flux that is shielded by the aperture, depth of focus can be appropriately maintained in all magnifications by changing the opening area substantially linearly in accordance with the magnification.
A third aspect of the present invention is an image recording exposure device for recording an image on an exposure surface by simultaneously carrying out multiple main-scans on the exposure surface, the device comprising: light sources arranged in a row in a sub-scanning direction and capable emitting multiple light beams simultaneously; exposure lenses including multiple lenses for focusing light beams emitted from the light sources onto the exposure surface along an optical path, at least two exposure lenses being disposed at positions different from one another on the optical path; a magnification changer for changing magnification of the exposure lenses; and an aperture disposed between the positions at or in the vicinity position at which a far-field pattern is formed by the emitted light beams, the aperture having an opening of constant area which transmits only a portion of luminous flux of the light beams therethrough, wherein the exposure lenses include lenses for changing the magnification disposed between the aperture and the light sources.
The third aspect of the present invention is an image recording exposure device to which the optical system for scanning according to the second aspect of the present invention has been applied. This aspect of the invention has the same operation and effects as those of the first aspect and the second aspect of the present invention (see FIG. 1 and FIG. 2).
The magnification is changed when resolution is to be changed. For example, if the resolution in the sub-scanning direction is doubled, the speed in the sub-scanning direction is xc2xd and if the light amount is also xc2xd, the amount of light which is exposed onto the scan-surface does not change. Consequently, no problem of insufficient light amounts is caused by the arrangement of the aperture.
Of the group of lenses, a lens system for changing the magnification is arranged between the aperture and the light source. Accordingly, the region of the luminous flux is not changed, and so the opening area of the aperture can be made constant.
A fourth aspect of the present invention is an image recording exposure device for recording an image on an exposure surface by simultaneously carrying out multiple main-scans on the exposure surface, the device comprising: light sources arranged in a row in a sub-scanning direction and capable emitting multiple light beams simultaneously; exposure lenses including multiple lenses for focusing light beams emitted from the light sources onto the exposure surface along an optical path, the exposure lenses including at least two lenses disposed at positions different from one another on the optical path; a magnification changer for changing magnification of the exposure lenses; and an aperture disposed between the positions at or in the vicinity position at which a far-field pattern is formed by the emitted light beams, the aperture having an opening which transmits only a portion of luminous flux of the light beams therethrough, and an opening area changer which changes area of the opening of the aperture in accordance with magnification, wherein the exposure lenses include lenses for changing the magnification disposed between the aperture and the exposure surface.
In accordance with the fourth aspect of the present invention, of the group of lenses, the lens system for changing the magnification is arranged between the aperture and the exposure surface. Accordingly, by using the opening area changing means to change the opening area of the aperture substantially linearly in accordance with the specified magnification, the opening area is changed in accordance with the magnification, and so an appropriate depth of focus can be maintained for all magnifications.
The opening area changing means may be a mechanism which mechanically works together with the magnification changing means.
There is an interrelationship between the amounts by which the magnification of the lens system and the opening area of the aperture are changed. Therefore, the lens system and the opening area of the aperture can be made to both change together, for example, by mechanically interlinking with gears of a specified gear ratio.